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8898 



Bureau of Mines Information Circular/1982 




Site-Specific and Regional Geologic 
Considerations for Coalbed 
Gas Drainage 



By W. P. Diamond 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8898 



Site-Specific and Regional Geologic 
Considerations for Coalbed 
Gas Drainage 



By W. P. Diamond 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



IN ass 



This publication has been cataloged as follows: 



Diamond, W. P. (William P.) 

Site-specific and regional geologic considerations for coalbed gas 
drainage. 

(Information circular ; 8898) 

Bibliography: p. 22-24. 

Supt. of Docs, no.: I 28.27:8898. 

1. Coal mines and mining— United States— Safety measures. 2. 
Boring. 3. Coal— Geology— United States. 4. Methane. I. Title. II. 
Series: Information circular (United States. Bureau of Mines) ; 8898. 



TN2&5.U4 622s [622\8] 82-600254 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments • 3 

The coalbed as a gas reservoir 4 

Site-specific considerations 5 

Direct method determination of the gas content of coal 5 

Sampling 5 

Tes t equipment 8 

Calculation of gas content 9 

Auxiliary test procedures 12 

Geologic considerations 13 

Variations in gas content 13 

Coalbed discontinuities 14 

Multiple coalbed reservoirs 17 

Regional considerations 19 

Calculation of an area ' s in-place gas volume 19 

Additional regional considerations 22 

Conclusions 22 

References 22 

ILLUSTRATIONS 

1. Coalfields of the United States 3 

2. Comparison of the gas storage potential of coal and 10-pct-porosity non- 

reactive reservoir rock versus reservoir pressure 4 

3. Gas content of coal versus actual mine emissions 5 

4 . Conventional and wire line coring equipment 6 

5 . Coalbed correlation problems 7 

6. Variable distribution of coalbeds in three wells, Trinidad, CO 8 

7. Sample containers used for direct-method testing of coal samples 9 

8. Equipment for direct-method testing of coal sample 9 

9. Lost-gas graph 11 

10. Gas content versus depth for the Mary Lee Coalbed, Alabama 13 

11. Map of rank distribution and depth distribution of the Mary Lee Coalbed, 

Alabama 15 

12. Section view of ideal coalbed and effect of coalbed discontinuities on 

horizontal gas drainage boreholes 15 

13. Section view of effect of coalbed discontinuities on vertical gas drain- 

age boreholes 16 

14. Examples of multiple coalbed reservoirs 18 

15. Isopach of the Mary Lee Coal Group superimposed on the overburden isopach. 20 

TABLES 

1. Estimates of total in-place gas volumes for U.S. coalbeds 2 

2. Highest measured gas contents of U.S. coalbeds 2 

3. States with highest measured gas emissions from coal mines 3 

4. Data for lost-gas graph 10 

5. Classification of coal by rank 14 

6 . In-place gas volume for the Mary Lee Coal Group , Alabama 20 

7. In-place gas volumes of selected U.S. coalbeds 21 



SITE-SPECIFIC AND REGIONAL GEOLOGIC CONSIDERATIONS 
FOR COALBED GAS DRAINAGE 

By W. P. Diamond 1 



ABSTRACT 

The Bureau of Mines has been involved in the drilling of vertical, 
horizontal, and directional coalbed gas drainage boreholes for mine 
safety since 1964. In that time, boreholes have been drilled in most of 
the major coal regions of the United States under a wide variety of ge- 
ologic conditions. Many of the geologic conditions that occur in the 
coal measures are detrimental to gas drainage; others may be beneficial. 
Analytical techniques to determine the gas content of coal samples and 
evaluate regional trends of gas distribution have been developed. 
Drilling techniques that maximize the acquisition of coalbed gas data 
and geologic information have been determined. 

Although some of the geologic factors influencing the placement and 
potential success of coalbed gas drainage boreholes have been reported 
in papers on individual projects, a complete, systematic compilation has 
not previously been available. The objective of this paper is to pro- 
vide information on specific geologic factors that should be considered 
prior to, during, and after the drilling of coalbed gas drainage bore- 
holes. Many of the commonsense considerations that have been learned 
through many years of Bureau of Mines experience, but have generally 
not been reported formally, are included for those who may be con- 
sidering coalbed gas drainage drilling for the first time, or who have 
not had the opportunity to encounter a substantial number of geologic 
situations. 



^Supervisory geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, 
PA. 



INTRODUCTION 



The Bureau of Mines has been investi- 
gating the occurrence of gas in coal and 
techniques to remove the gas in advance 
of mining since 1964 (28). 2 The goal of 
the Bureau's research program has primar- 
ily been to increase mine safety by re- 
ducing the explosion hazard of methane- 
air mixtures. Many of the coalbed gas 
observations and techniques developed 
have applications both for mine safety 
and for energy resource delineation and 
utilization. The evaluation procedures 
and geologic considerations for drilling 
sites discussed in this paper are rel- 
evant to both mine safety and gas utili- 
zation programs. 

It is estimated that coalbeds in the 
United States contain as much as 21.7 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



trillion m 3 (766 trillion ft 3 ) of in- 
place gas (table 1). The gas is 
distributed in varying unit volumes 
throughout the extensive coal reserves of 
the United States (fig. 1). Gas contents 
ranging from essentially 0.0 cm 3 /g (0.0 
ft 3 /ton) to 21.6 cm 3 /g (691 ft 3 /ton) have 
been measured. Table 2 is a list of the 
highest measured gas contents of U.S. 
coalbeds. A list of 583 gas content 
tests on 125 coalbeds in 15 States can be 
found in Bureau of Mines RI 8515 (8). 

TABLE 1. - Estimates of total in-place 
methane volumes for U.S. coalbeds 



Source 


Trillion 


Trillion 




m 3 


ft 3 


Bureau of Mines (6).. 


21.7 


766 


National Energy 








1.4-19.8 


50-700 


National Petroleum 








11.2 


398 



TABLE 2. - Highest measured gas contents of U.S. coalbeds 



Coalbed or formation 



County and State 



Depth 



m 



ft 



Gas content 



cm 



Vg 



IWi 



on 



Coal rank 



Peach Mountain 

Pocahontas No. 3.... 



Tunnel...., 
New Castle, 
Mary Lee. . , 



Hartshorne. . , 
Me s aver de Fm, 

Beckley , 

Vermej o Fm. . , 



Schuylkill, PA. 
Buchanan, VA. . . 

Schuylkill, PA. 
Tuscaloosa, AL. 
do 



Pratt, 



Le Flore, OK.... 
Sublette, WY.... 

Raleigh, WV 

Las Animas, CO.. 
Tuscaloosa, AL. . 



209 
568 

185 

650 
666 

439 

1,065 

253 

547 

416 



685 
1,864 

608 
2,132 
2,185 

1,439 

3,495 

830 

1,793 

1,365 



21.6 
21.5 

18.3 
17.5 
17.4 

17.1 

17.0 

15.3 

15.3 

15.1 



691 
688 

586 
560 
557 

547 

544 

490 

490 

483 



Anthracite. 
Low-volatile 
bituminous. 
Anthracite. 

Low-volatile. 

bituminous. 
Medium-volatile 

bituminous. 
High-volatile A 

bituminous. 
Medium-volatile 

bituminous. 
Do. 



An indirect measure of the possible 
safety hazard of methane in coal mines, 
as well as the resource potential of 
coalbed methane, is the volume of methane 
vented from U.S. coal mines. As of the 
last survey by the Bureau of Mines in 
1975 (L5), over 5.7 million m 3 (200 mil- 
lion ft 3 ) of methane per day was being 
vented. The seven States with the high- 
est methane emissions are listed in ta- 
ble 3. Sixty individual mines vented 
0.03 million m 3 (1 million ft 3 ) or more 
per day. 



TABLE 3. - States with highest measured 
gas emissions from coal mines (1975) 



State 



West Virginia, 
Pennsylvania , 

Virginia , 

Alabama 

Illinois 

Colorado , 

Ohio 



Gas Emissions 



Million 


Million 


m 3 /d 


ft 3 /d 


2.7 


96.4 


1.2 


43.1 


.6 


22.1 


.5 


16.6 


.4 


14.7 


.3 


9.1 


.2 


6.1 



ACKNOWLEDGMENTS 



Appreciation is extended to Arie M. 
Verrips, Executive Director, American 
Public Gas Association (APGA) , for 
providing data from the Unconventional 
Gas Recovery and Utilization drilling 



program. Carol Tremain and Donna Boreck, 
geologists, Colorado Geological Survey, 
are acknowledged for providing geologic 
correlations for the APGA wells at Trini- 
dad, CO. 




ALASKA 



Medium-and high-volatile 
bituminous coal 



i Low-volatile bituminous coal 

] Anthracite and semianthracite 
coal 



FIGURE 1. - Coalfields of the United States. 



THE COALBED AS A GAS RESERVOIR 



The fundamental principle to accept 
when considering coalbeds as gas reser- 
voirs is that they are not the same as 
as "traditional" gas reservoirs (such as 
sandstones) and do not behave in accord- 
ance with the same reservoir mechanics. 
In a traditional sandstone reservoir, the 
gas exists as free gas in the void spaces 
between sand grains, and transport of 
that gas through the reservoir is gov- 
erned by pressure gradients as described 
by Darcy's law. 



The attractiveness of coalbeds for com- 
mercial gas production is illustrated in 
figure 2, which compares the theoretical 
gas volumes that can be stored at various 
pressures in equal rock volumes of both 
traditional reservoirs of 10-pct porosity 
and representative coalbeds. Even at the 
relatively low reservoir pressures com- 
monly found in coalbeds, the unit volume 
of coal can store several times the gas 
volume of the 10-pct porosity nonreactive 
traditional reservoir rock. 



In a virgin coalbed reservoir, only a 
small portion of the methane is found as 
"free" gas in the fractures (cleat). 
Most of the methane is adsorbed on the 
coal surface in the extensive micropore 
structure of the coal (_5). The transport 
of the methane from the micropores 
through the "solid" coal is governed by 
concentration gradients as described by 
Fick's law of diffusion. Once the meth- 
ane has reached the coalbed fracture sys- 
tem, the transport of the gas through the 
cleat to a well bore or mine opening is 
governed by Darcy's law. 

In a virgin coalbed reservoir, the 
pressure in the fracture system and the 
concentration of methane in the micropore 
structure are in equilibrium (5). To in- 
duce a flow of methane from the solid 
coal, the equilibrium must be disrupted 
by lowering the pressure in the fracture 
system. Coalbeds are generally saturated 
with water, which when removed, either by 
pumping from a vertical borehole or by 
"natural" drainage into a mine opening or 
horizontal borehole, disrupts the equi- 
librium. Methane can then desorb from 
the coal micropores and is made available 
for flow through the fracture system. 
Gas flows will normally continue as long 
as equilibrium conditions are disrupted 
but can quickly decline if equilibrium is 
reestablished. The continued lowering of 
the coalbed reservoir pressure to ensure 
gas flow is completely opposite to the 
situation with a traditional reservoir, 
where maintaining a high reservoir pres- 
sure is required to provide the energy to 
flow large volumes of gas. 



An additional factor to be considered 
for coalbed reservoirs is the influence 
of "boundaries" on the reservoir and gas 
flow rates. A boundary, either natural 
(such as a fault) or created (interfer- 
ence from other boreholes or mine open- 
ings ) , can reduce the time needed to 
lower the coalbed pressure and induce gas 
flows (_3, 21). The boundaries effec- 
tively limit the size of the reservoir, 
and the pressure is more efficiently re- 
duced in the resulting area, with an 
accompanying decrease in the time needed 



35 



T 




_L 



Nonreactive n 
10-pct porosity 

I L 



400 800 1,200 1,600 
PRESSURE, psig 



2,000 2,400 



FIGURE 2. - Comparison of the gas storage potential 
of coal and 10-pct-porosity nonreactive reser- 
voir rock versus reservoir pressure (26). 



to achieve gas production. Several 
publications (3, _5> 20-21) detail the 
reservoir characteristics of coalbeds and 



provide mathematical descriptions 
reservoir mechanics. 



of the 



SITE-SPECIFIC CONSIDERATIONS 



Site-specific considerations of the 
methane potential of coalbeds include 
both determining the in-place gas content 
and identifying geologic factors that may 
affect the flow of gas from the coalbed 
reservoir to methane drainage systems or 
underground mines. 

Direct Method Determination of the Gas 
Content of Coal 

The Bureau of Mines originally became 
interested in determining the gas content 
of virgin coal as an aid in estimating 
the amount of gas that would be released 
in active underground mines. The initial 
research results were used to construct a 
graph (fig. 3) that related direct method 
test values to the actual measured meth- 
ane emissions of nearby mines. 

Sampling 

Coal samples for determination of gas 
content are obtained either from continu- 
ous wire line core holes or from rotary- 
drilled boreholes by means of con- 
ventional coring of selected zones. 
Schematic diagrams of wire line and con- 
ventional types of coring equipment are 
shown in figure 4. In general, the con- 
tinuous wire line technique is preferred 
for obtaining coal samples for gas con- 
tent determination. The time required to 
remove the coal sample from the hole and 
seal it into a desorption container is 
important for good test results. (See 
"Calculation of Gas Content" section.) 
The retrieval of samples by the wire line 
coring technique is very fast since the 
inner barrel containing the coal is 
brought through the drill pipe to the 
surface by the wire line without having 
to pull the entire string of drill pipe 
from the hole, as with conventional cor- 
ing equipment. The difference in re- 
trieval time can be several hours at 
depths greater than 305 m (1,000 ft). 



An additional problem that is fre- 
quently encountered with the conventional 
coring of selected zones is missing the 
coalbed that is to be cored, or coring 
excessive lengths of section while 
searching for the coal. In the conven- 
tional coring technique, the hole is 
rotary-drilled (no core taken) to a point 
(depth) estimated to be near the top of 
the coalbed; then the drill pipe is 
pulled from the hole, the rotary drill 
bit is taken off, and the core barrel is 
installed. The core barrel is then run 
into the hole on the end of the drill 



3.2 



2.8 



ro 



o 24 
o 



y 2.0 



o 

or 



o 
in 

UJ 

< 



< 

I- 
o 
< 



.6 



.8 



.4 



Beatrice MinejQ 



7 



Loveridge 
Mine-%^ 



<Jff"< 



/ 



/ 



7 



CfHowe Mine 



Federal No.2 Mine 



2 - 



/ 



/ 



Qt— Vesta Mine 

~J Mary Lee No. I Mine 

~&w__^y Inland Mine 

r7\ L__i i i i_ 



J_ 



4 8 12 16 20 

GAS CONTENT MEASURED 

BY DIRECT METHOD, cm 3 /g 

FIGURE 3. - Gas content of coal versus actual 
mine emissions. 



il 



Drill pipe 



■Rock core 



■Inner tube 




Wire line 
Drill rod 

Overshot 



-Rock core 



Inner tube 



S— — Bit 



FIGURE 4. - A, Conventional, and B, wire line 
coring equipment. 

pipe, and hopefully the position of the 
coalbed has been estimated with suffi- 
cient accuracy to have the entire coalbed 
in a single run of the barrel, which is 
typically 3.05 to 6.10 m (10 to 20 ft) 
in length. 

Figure 5 illustrates several of the 
problems that can be encountered in 
attempting to predict (correlate) where 
coalbeds should be encountered if suffi- 
cient geologic data are not available or 
if the geologic section is variable. 
Commonly a nearby data source, such as a 
previously drilled well or regional geo- 
logic trends, is used to determine the 
core points where the conventional core 
barrel will be installed to obtain sam- 
ples. Usually core points are projected 



using marker beds and interval thick- 
nesses. In the example in figure 5, a 
marker bed from a nearby, previously 
drilled hole (well A) has been identified 
while rotary drilling well B. The inter- 
val between the bottom of the marker bed 
in well A and the top of coalbed A is 
known and is used to project a core point 
(I) in the new hole, well B. Unfortu- 
nately, the interval has thinned towards 
well B, putting coalbed A above the pro- 
jected core point. If the driller were 
given only the depth below the marker bed 
as guidance for the core point, it is 
quite likely that the coalbed would be 
drilled through and no core obtained. 

Once it has been found that the inter- 
val between the marker bed and coalbed A 
has thinned in well B, a decision must be 
made on the core point to obtain coalbed 
B. Several options are available: 

1. Assume that the position of coalbed 
B has no relationship to the coalbeds in 
well A, and conventional-core the entire 
interval below coalbed A until coalbed B 
is obtained, which could be very time 
consuming and expensive. 

2. Assume that the intervals between 
coalbed A and coalbed B are the same in 
wells A and B, which would put the core 
point (II) in the correct position to 
obtain a sample. 

3. Assume that the intervals between 
the marker bed and coalbed B are the same 
in wells A and B, which would put the 
core point (III) below the actual posi- 
tion of coalbed B, and the coalbed would 
probably be missed for coring. 

4. Assume that the interval between 
coalbeds A and B in well B thins by the 
same amount as the interval between the 
marker bed and coalbed A in the same 
well, which would put the core point at a 
depth slightly below the protected core 
point (I)> requiring unnecessary and 
expensive coring before encountering 
coalbed B. 



Wei 



Marker bed 



'■ ■■■.■■ ' ■ ' . ^ 



Coalbed A 



Coal bed B 



Correlation 

Projected core point 



Coalbed C and rider 



A 



Wei 



? QPi. 



£ 



Marker bed 



■ ■■ '■ ' ■ ' ■ 'i 1 ' 1 



Coalbed A 



Coalbed B 



Coalbed C 



FIGURE 5. - Coalbed correlation problems. 



Coalbeds are frequently overlain by 
thin rider coals which can be used as 
marker beds for conventional core points. 
Coalbed C in well A has a rider coal 
above it; however, the rider is not pres- 
ent in well B. If the driller on well B 
had been told to stop when the rotary 
drilled into the first coalbed below 
coalbed B (supposedly coalbed C rider) 
and switch to the core barrel to core the 
main coalbed, part of coalbed C would 
have been penetrated and the coal lost. 
Many of the situations described above 
for picking core points for conventional 
coring would result in a loss of coal for 
gas content determinations and/or addi- 
tional unprogrammed expense for excessive 
core drilling. The use of continuous 
wire line coring would eliminate or mini- 
mize the described problems. 

In unknown geologic areas where wire 
line coring cannot be used for reasons 
such as cost or lack of suitable drilling 
rigs, recovery of all coalbeds using con- 
ventional coring can be enhanced by 
"twinning." A rotary hole is first 



drilled through the entire section of 
interest, without taking any cores. The 
hole is then logged with geophysical 
equipment to precisely define the depth 
and thickness of the coalbeds. The 
drilling rig is then moved over a few 
meters (feet), and a second ("twin") hole 
is rotary-drilled. Since the exact loca- 
tion of each coalbed is known from the 
first hole, the conventional core barrel 
can be used at core points precisely 
located above each coal. The variability 
in distribution of coalbeds over a small 
geographic area is illustrated with an 
example from a drilling project near 
Trinidad, CO (fig. 6), where the wells 
are approximately 150 m (500 ft) apart. 

Multiple testing, or preferably testing 
of the entire coalbed, is the preferred 
sampling strategy. Variations in gas 
content are commonly observed on multiple 
samples from the same coalbed in a core 
hole. A single test on a small portion 
of a coalbed may yield a falsely low or 
high value for the entire coalbed. 



Ae g 



Well 3 



BULK DENSITY, BULK DENSITY, BULK DENSITY, 

sdu sdu sdu 

6,000 6,000 6,000 

5,8501 r 



5,800 



5,750 - 



5,700- 



t 5,650 



™ 5,600 
Q 



5,550 



5,500 



5,450 



5,400 - 



5,350 



-5 



Zone 
D 



Zone 
C 



Zone 
B 



J 



Zone 
C 



Zone 
5 



-Zone 
A 



J 



Zone 
D 



Zone 



LEGEND 



^ 



,Coal or 
bone 



Zone 

B sdu Standard 
density unit 



r-Zone 
- J A 



Las Animas^ 



Key mop 
3,000 



Scale, ft 



FIGURE 6. - Variable distribution of coalbeds in 
three wells, Trinidad, CO. (Correlations 
courtesy of Colorado Geological Survey.) 

Test Equipment 

Sample containers of several shapes and 
sizes that have been constructed for var- 
ious testing purposes are shown in fig- 
ure 7. The standard container (can A) 
used by the Bureau is made from a 0.3-m 
(1-ft) piece of aluminum pipe, having an 
inside diameter of 10 cm (4 in). A 
top flange and bottom plate have been 
welded to the pipe section, and a remov- 
able lid that attaches to the top flange 
can be fitted with a gage and various 
types of valve assemblies. Valves with a 
quick-connect capability are preferred 



for convenience and time savings if a 
large number of samples are tested at the 
same time. 

Less expensive alternatives to the 
metal canisters are the various plastic 
water filter housings (cans B, C, and D) 
available from many plumbing supply out- 
lets. These containers are sometimes 
awkward to use because of their rounded 
bottoms (cans C and D), or because of the 
difficulty of opening and/or sealing the 
large screw-type caps. Thus, standard 
metal containers are preferred because of 
their flat bottoms and durability, espe- 
cially in long-term collection programs. 
In general, any container that can be 
easily sealed airtight, can contain about 
2 kg (4.4 lb) of sample, and can hold 
approximately 414 kPa (50 lb/in^ g) of 
internal pressure would be adequate for 
the test. 

It has been suggested that containers 
of greater length, perhaps even long 
enough to hold an entire core of an coal- 
bed, should be used for testing. Al- 
though it would be preferable to test the 
entire core, several complications may 
arise in using large containers. Occa- 
sionally, a sample container will leak, 
invalidating the test. If six individual 
0.3-m (1-ft) sections of a 1.8-m (6-ft) 
coalbed are tested separately, a leak in 
one can is of little consequence. But if 
the entire 1.8 m (6 ft) is placed in one 
can and it leaks, little usable data may 
be obtained. Furthermore, coal samples 
that are friable and very gassy will usu- 
ally give off large volumes of gas early 
in the desorption procedure. If very 
large amounts of coal of this type are 
sealed into a large canister, then bleed- 
ing the large volume of gas into the mea- 
suring apparatus, which will be described 
later, can require an excessive amount of 
time (several minutes). Long measuring 
times may invalidate the calculation of 
the lost gas, which requires graphing of 
gas volumes at instantaneous points in 
time. 

The equipment (fig. 8) needed to mea- 
sure the actual volume of gas desorbing 




""•""ima i W~ 



FIGURE 7. - Sample containers used for direct-method testing of coal samples. Can A-standard 
container; cans B, C, and D-plastic water filter containers. 



from the coal sample consists of an in- 
verted graduated cylinder sitting in a 
pan filled with water and a ring stand 



Valve 
30-Psi gage^T7)\ 



/Inverted graduated 
cylinder 




Sample 
container 



Tube 




i3= 



#= 



Clamp 
stand 



b^ E 



Pan of water 



FIGURE 8. - Equipment for direct-method test- 
ing of coal sample. 



and clamps to hold the graduated cylinder 
in place. The desorbed gas that collects 
in the canister is periodically bled into 
the graduated cylinder and measured as 
the volume of water displaced. This pro- 
cedure is performed both at the drill 
site and subsequently in the laboratory. 

Calculation of Gas Content 

The gas content of a particular sample 
is composed of lost, desorbed, and resid- 
ual gas, each of which is determined by 
slightly different techniques. A core 
sample actually begins to desorb gas 
before it is sealed in the sample 
container. The amount o£ this lost gas 
depends on the drilling medium and the 
time required to retrieve, measure, and 
describe the core, and seal the sample in 
the can. The shorter the time required 
to collect the sample and seal it into 



10 



the can, the greater the confidence in 
the lost-gas calculation. As discussed 
previously, because of its speed, wire 
line retrieval of the core is preferable 
to conventional coring. If air or mist 
is used in drilling, it is assumed that 
the coal begins desorbing gas immediately 
upon penetration by the core barrel. 
With water, desorption is assumed to be- 
gin when the core is halfway out of the 
hole; that is, when the gas pressure is 
assumed to exceed that of the hydrostatic 
head. 



Time core reached surface (C) — 
12:40 a.m. 

Time core sealed in canister (D)- 
12:50 a.m. 

Lost gas time: 

(D-A) if air or mist is used 



C— B 
(D-C) + — — if water is used 



The lost gas can be calculated by 
a graphical method based on the rela- 
tionship that for the first few hours 
of emission, the volume of gas given 
off is proportional to the square 
root of the desorption time. A . plot 
of the cumulative emission after each 
reading against the square root of the 
time that the sample has been desorb- 
ing ideally would produce a straight 
line. 

A sample of experimental data (ta- 
ble 4) and supplementary information 
used to construct a lost-gas graph 
follows: 

Drilling medium — water. 

Time coalbed encountered (A) — 
12:01 a.m. 

Time core started out of hole (B) — 
12:30 a.m. 



(12:40-12:30) 



(12:50-12:40) + 



= io + M 



=15 minutes. 

The resulting graph is 
ure 9. The intercept on 
the square root of the 
(lost-gas time) in minutes 
gas desorption begins and 
sealed in the container, 
value of the lost gas is 
which the constructed line 
the negative Y axis. 



shown in fig- 

the X axis is 

elapsed time 

from the time 

the sample is 

The estimated 

the point at 

intercepts the 



The desorbed gas is simply the total 
volume of gas drained from the sample and 
measured in the graduated cylinder. The 
desorbing of a sample is generally 



TABLE 4. - Data for lost-gas graph 





Time , 
a.m. 


Time since 
placed in 
can, min 


/ Time in 

V can+15, 

min 1 /2 


Gas 


released 


Total gas 


Reading No . 


cm^ 


10 _J ft* 


cm ^ 


10 _;i ft^ 


1 


12:50 
1:05 
1:20 
1:35 
1:50 
2:05 
2:20 



15 

30 
45 
60 
75 
90 


3.87 
5.48 
6.71 
7.75 
8.66 
9.49 
10.25 



92 
84 
55 
36 
40 
33 



3.25 
2.97 
1.94 
1.30 
1.41 
1.17 



92 
176 
231 
267 
307 
340 





2 


3.25 


3 


6.22 


4 


8.16 


5 


9.46 


6. 


10.87 


7 


12.04 



11 



ro 

E 
o 

O 
O 

#» 
CO 

< 

CD 

Q 
LU 
CD 
QC 
O 
CO 
LU 
Q 



4 I — i — i — i — i — i i r 







-2 



-3 



vnr 




/ 



A- — Projection 



/ 
/ 

-/ 

/^ Lost gas = 240 crrr 



_L 



J_ 



-L 



J_ 



X 



2 4 6 8 10 

1 

VflME, min 2 

FIGURE 9. - Lost-gas graph, 



12 



allowed to continue until a very low 
emission rate is obtained, generally an 
average of less than 10 cm 3 (0.35 x 10~ 3 
ft 3 ) of gas per day for 1 week. The time 
required to reach this low rate of 
emission will vary considerably and is 
affected by many things, including the 
size of the sample, the physical charac- 
teristics of the coal, and the amount of 
gas contained in the sample. 

When it is determined to discontinue 
the measurement of desorbed gas, the coal 
sample will usually still contain gas. 
To complete the gas determination proce- 
dure, the amount of residual gas must be 
measured. The procedure recommended by 
the Bureau is to crush the coal in a 
sealed ball mill. The ball mill con- 
structed for crushing coal was fabricated 
from a piece of 0.64-cm (1/4-in) wall, 
17.78-cm (7-in) diameter steel pipe. A 
steel plate was welded to the bottom, and 
a lid was fitted to the top. At the top, 
a short section of pipe with 2.54-cm 
(1-in) wall thickness was welded inside 
the 17.78-cm (7-in) pipe to provide suf- 
ficient surface area for machining a 
groove for an 0-ring seal and for bolt 
holes to secure the lid. 

The ball mill is tumbled on a roller 
machine for approximately 1 hr to crush 
the coal. The mill is allowed to cool to 
room temperature, and the volume of gas 
released is then measured by the water 
displacement method. The crushed powder 
and any uncrushed lumps are weighed sepa- 
rately. The volume of gas released is 
attributed only to the crushed powder. A 
set of residual gas data and calculation 
procedure follows: 

Weight of crushed powder — 735 g 



Residual gas calculation = 



Weight of uncrushed lumps — 45 g 

Volume of gas bleed off — 1,082 cm 3 
gas bleed off, cm 3 



weight of sample crushed to powder, g 

1,082 cm 3 
735 g 

= 1.5 cm 3 /g (24.02 x 10" 3 ft 3 /lb, 48 ft/ton). 



12 



Theoretically, it is possible to crush 
a coal sample in the ball mill at any 
point after collection and to obtain the 
total gas content (excluding lost gas) of 
the sample. This procedure is generally 
not considered appropriate if maximum in- 
formation from the sample is desired. By 
crushing the sample before the desorption 
process is complete, it is impossible to 
obtain the relative amounts of desorbed 
and residual gas. This distinction is 
important because the actual residual 
gas, which will not desorb from the sam- 
ple while sealed in the canister, prob- 
ably represents gas that will not flow to 
a methane drainage borehole and possibly 
represents gas that will not be emitted 
into a mine atmosphere. It is true 
that during the process of mining coal, 
the coal is broken up into variously 
sized pieces; however, the majority of 
these pieces will not usually dupli- 
cate the very fine powder that the ball 
mill produces in the residual gas 
procedure. 

The total gas content of a particular 
sample is the volume of lost gas and de- 
sorbed gas divided by the total sample 
weight plus the residual gas content. 
The calculation procedure and sample data 
set follow: 

Lost gas — 240 cm 3 

Total gas = lost .^jl^sorbedjgas 
total sample weight 
+ residual gas 

240 cm 3 + 3,246 cm 3 

780 g 
+ 1.5 cm 3 /g 

= 4.5 + 1.5 

4- 6.0 cm 3 /g (96.10xl0~ 3 ft 3 /lb, 
192 ft 3 /ton) 

Auxiliary Test Procedures 

Proximate, ultimate, and Btu analyses 
are obtained on the crushed powder from 
the residual gas test. These test re- 
sults can be used to further evaluate the 
gas content results on a practical and 
theoretical basis. 



Because the gas content is presented on 
a volume-to-weight ratio, the presence of 
noncoal material, primarily shale and 
pyrite (which adds weight but not gas 
storage capacity), can produce seemingly 
erroneous data. Thus two samples from 
the same coalbed core may have gas con- 
tents varying by several cubic centi- 
meters per gram if one sample contains 
appreciably higher noncoal material. The 
coal analysis will help determine if non- 
coal material is influencing the total 
gas content. 

Evaluation of the influence of depth of 
burial on the gas content is preferably 
done on a clean coal, thus removing the 
noncoal material variable from the evalu- 
ation. However, because coalbeds do con- 
tain noncoal material, the actual in- 
place methane in a particular volume of 
coal should be related to the as-received 
coal data. 

Theoretically, the gas content of coal 
is influenced by the rank of the coal, 
with higher ranks generally having higher 
gas contents. The coal analysis can be 
used to determine the apparent rank of 
the coal by ASTM Standard D388 (_2) for 
evaluation of the rank parameter. Coal 
petrography, specifically vitrinite re- 
flectance measurements, can also provide 
a measurement of coal rank. Determining 
the microscopic constituents of the coal 
(macerals) may also be useful in investi- 
gations of the factors influencing the 
methane content of coalbeds. Adsorption 
isotherm tests ( 18 ) will give data on the 
theoretical storage capacity of a sample, 
and along with the other analytical 
tests, can be an important tool for eval- 
uating the direct method test results. 

Gas samples should be obtained peri- 
odically during the desorption testing of 
coal samples. Gas compositional analysis 
will provide information on the gas qual- 
ity, including the presence of gases 
other than hydrocarbons. Ethane, pro- 
pane, and butane are common hydrocarbons 
found in small amounts (generally less 
than 2 pet combined) in coalbed gas. 
Carbon dioxide (occasionally in amounts 
as high as 15 pet) is a common, poten- 
tially undesirable, component of coalbed 



13 



gas which has been found at the higher 
levels primarily in a relatively small 
area of the Pittsburgh Coalbed in Penn- 
sylvania and West Virginia, and in sev- 
eral western coals. Several publications 
(17, 19, 29) discuss the origin and com- 
position of coalbed gas. 

Geologic Considerations 

Variations in Gas Content 

The gas content of individual coalbeds 
has been observed to increase as the 
depth of the coalbed increased (9-10, 13- 
14 , 25 , 27 , 30). A coalbed contains more 
gas at greater depths primarily owing to 
the increase in reservoir pressure, which 
allows the coal to "hold" more gas, if it 
is available. Figure 10 illustrates in- 
creasing gas content with increasing 
depth for the Mary Lee Coalbed in Ala- 
bama. It is important to note that this 
graph is only for the Mary Lee Coalbed 
and should not be used to estimate the 
gas content of any other coalbed or coal 
region. 

Even though the gas content of a coal- 
bed increases with depth, this does 
not mean that all deep coalbeds are 



necessarily gassy . Many coalbeds in the 
Eastern United States contain appreciable 
gas (>5 cm 3 /g [160 ft 3 /ton]) at depths of 
305 m (1,000 ft) (8). However, many 
coalbeds in the western United States at 
depths of 305 m (1,000 ft) contain little 
gas (<2 cm 3 /g [64 ft 3 /ton]). The reason 
for these variations in gas contents of 
coalbeds at similar depths are in many 
cases due to differences in the rank of 
the coal. Coal rank (table 5) is a mea- 
sure of the stage of coalif ication that a 
coal deposit has reached. The coalifica- 
tion process progressively transforms the 
original plant material into higher ranks 
of coal, depending primarily on tempera- 
ture and time and to a lesser extent on 
pressure (29) . Methane is generated 
throughout the coalif ication process in 
varying amounts, with an increased yield 
of methane associated with reduction of 
hydrogen, which begins at approximately 
29 pet volatile matter content in the 
medium-volatile bituminous coal range 
(29). Owing to the influence of the 
coalif ication process, it is therefore 
possible to have deep coalbeds that have 
not gone through the stages that produce 
high volumes of methane and that do not 
have high gas contents. 



c 
o 



ro 



700 



600 



500 - 



l-T 400 - 

LU 

t; 300 

o 
o 

</> 200 
< 

CD 

100 



1 


1 ' 1 ' 


i ' i 


i 


— 


_ 






• 


- 





• 






— 




• 


- 


— 








— 


— 


/ 
/ 






— 




/ ^""^Inferred 

t 

i . i i . i 


1 1 1 


1 


— 



- 20 



- 16 o> 

E 
o 

LU 

I- 



- 4 
t 

I I I I I I I I i 

500 1,000 1,500 2,000 2,500 

DEPTH, ft 

FIGURE 10. - Gas content versus depth for the Mary Lee Coalbed, Alabama. 



< 



14 



TABLE 5. - Classification of coal by rank (2) 



Group 



Fixed carbon 
limits, pet 
(dry, mineral- 
matter-free basis) 



Equal to 

or greater 

than — 



Less 
than — 



Volatile matter 
limits, pet 
(dry, mineral- 
matter-free bases) 



Greater 
than — 



Equal to 
or less 
than — 



Calorific value 
Btu/lb (moist, 
mineral-matter- 
free basis) 



Equal to or 
greater 
than — 



Less 
than— 



CLASS I.— ANTHRACITIC 



Meta-anthracite 

Anthracite 

Semi anthracit e. 

Low-volatile 

bituminous c 
Medium-volati 

bituminous c 
High-volatile 

bituminous c 
High-volatile 

bituminous c 
High-volatile 

bituminous c 

Subbituminous 
A coal 

Subbituminous 
B coal 

Subbituminous 
C coal 

Lignite A. . . . 
Lignite B.... 



98 
92 

86 



98 
92 



2 

8 

14 



CLASS II. —BITUMINOUS 



Low-volatile 
















78 


86 


14 


22 


• • • 


• • • 


Medium-volatile 
















69 


78 


22 


31 


• • • 


• • • 


High-volatile A 
















• • • 


l 69 


31 


• • • 


14,000 


• • • 


High-volatile B 
















• • • 


• • • 


• • • 


• • • 


13,000 


14,000 


High-volatile C 
















• • • 


• • • 


• • • 


• • • 


11,500 


13,000 



CLASS III.— SUBBITUMINOUS 



10,500 
9,500 
8,300 



11,500 

10,500 

9,500 



CLASS IV.— LIGNITIC 



6,300 



8,300 
6,300 



The graph (fig. 10) that is a plot of 
direct method gas contents versus depth 
of samples from the Mary Lee Coal Group 
is also influenced by rank variations of 
the coalbeds. Figure 11 shows the dis- 
tribution of coal rank and depth in the 
basin. In general, the rank increases 
with depth; however, this relationship is 
variable and does not precisely correlate 
throughout the area. Because of the high 
numbers of samples that would be needed 
for direct method testing to document the 
change in gas content with rank in a coal 
basin and the relative ease of mapping 
the changes in depth and obtaining sam- 
ples from a variety of depths for gas 
content determinations, the relationship 
of gas content to depth for coalbeds is 
most commonly presented. The Bureau of 
Mines is currently conducting research to 



document and relate the influence of coal 
rank as well as depth on gas content. 

Coalbed Discontinuities 

There are many geologic features that 
disrupt the continuity of a coalbed. 
They can be stratigraphic in origin and 
characterized by an interruption in sedi- 
mentation, either nondeposition or ero- 
sion, such as a sand channel; or they can 
be structural in origin and characterized 
by a surface separating two unrelated 
groups of rock, such as a fault (1). 
Discontinuities are an important con- 
sideration in evaluating the gas drainage 
potential of coalbeds. The presence of 
discontinuities can • cause serious prob- 
lems in the drilling and completion of 
both vertical and horizontal gas drainage 



15 



B 



BLOUNT 
— , COUNJY_ 

1 / 

]~"JEFFERSON 
J" COUNTY 




TUSCALOOSA i 
COUNTY 



Birmingham 



LEGEND 

' 7 ZA High volatile 

I 1 Medium volatile 

H^i Low volatile 

/O^ Contour interval is 
500 ft 



BLOUNT 
— COUNTY 




Birmingham 

40,000 m 



COUNTY 
FIGURE 11. - A, Map of rank distribution, and B, depth distribution of the Mary Lee Coalbed, Alabama. 



boreholes as well as influencing the flow 
of gas (decreasing, or in some cases in- 
creasing, production as previously dis- 
cussed) to the boreholes. 

An ideal coalbed from both a mining and 
a gas drainage standpoint would be of 
uniform thickness with no interruptions 
(fig. 12,4). This is seldom the case, as 



many coalbeds exhibit various types of 
stratigraphic discontinuities as shown in 
figure 12s. A horizontal gas drainage 
borehole drilled into this coalbed would 
probably encounter great difficulty both 
in drilling and in staying in the coal- 
bed. A vertical borehole (resource con- 
firmation corehole or production hole) 
would also experience problems in sample 




Splits 



Partings 




* Horizontal borehole 



FIGURE 12. - A, Section view of ideal coalbed, and B, effect of coalbed discontinuities on hori- 
zontal gas drainage boreholes. 



16 



recovery and gas flow if areas of thin or 
absent coal were encountered, as at the 
"roll" or in the area of the "splits." 

Boreholes drilled in an area of exten- 
sive partings (fig. 125) could encounter 
gas flow problems. A horizontal hole 
drilled completely above or below an ex- 
tensive parting that effectively sepa- 
rates a coalbed into separate reservoirs 
may drain methane only from that portion 
of the coalbed actually drilled. The 
other portion of the coalbed would remain 
undrained, and the gas would still be a 
hazard to future mining or would be un- 
available for commercial production. A 
stimulation treatment in a vertical hole 
drilled into an area with an extensive 
parting may not completely stimulate both 
portions of the reservoir. If the treat- 
ment did not efficiently penetrate above 
and below the parting, the gas flows 
could be reduced with the same potential 
consequences as described for the hori- 
zontal holes. 

Impermeable discontinuities that com- 
pletely disrupt a coalbed are particu- 
larly troublesome for gas drainage drill- 
ing activities. Figure 13 illustrates 
several geologic situations that can 

Borehole A 



Sand channel 



adversely affect drilling. Borehole A 
has been drilled into a sand channel and 
completely missed the coalbed. If this 
was a resource confirmation core hole, no 
coal would have been obtained for direct 
method gas content testing. If bore- 
hole A was for gas drainage, it would 
probably be ineffective unless gas had 
migrated (or would migrate) from the coal 
to the sand channel and was trapped. 
Clay veins are generally smaller than the 
sand channels, however, if they are en- 
countered, they can cause equally serious 
problems. 

Borehole B (fig. 13) has encountered a 
full section of the coalbed; however, it 
is bounded by a clay vein and a fault. 
This situation can be bad or good for gas 
production, depending on the size of the 
"cell" that borehole B has intercepted. 
If the bounded area is small, a limited 
amount of coalbed reservoir will be 
available to feed gas into the borehole, 
therefore, its production potential is 
low. If borehole B has penetrated a 
larger "cell" or is not completely 
bounded by discontinuities, the situation 
may enhance gas production. A bounded 
coalbed reservoir of this type will po- 
tentially have a faster pressure drawdown 



Borehole B 



Borehole C Borehole D 





Abandoned 
mine 




FIGURE 13. - Section view of effect of coalbed discontinuities on vertical gas drainage 
boreholes. 



17 



when dewatering is initiated, and higher 
gas saturations and production rates 
should follow as has been observed at a 
vertical borehole methane drainage pat- 
tern in Alabama (_3, 23). 

Borehole C (fig. 13) has completely 
missed the coalbed by intercepting a 
fault; therefore, there are no samples or 
gas production from the coal. Bore- 
hole D, which was to be a commercial 
well, has intercepted the coalbed; how- 
ever, it is very near an abandoned mine. 
An abandoned mine is not a natural coal- 
bed discontinuity, but it does interrupt 
the coalbed reservoir and can have seri- 
ous consequences. It is quite likely 
that in addition to a portion of the 
coalbed reservoir having been removed by 
mining, a significant amount of the gas 
originally in the remaining coal migrated 
to the mine openings and is no longer 
available for production from a borehole. 
Abandoned mines above the target coalbed 
must also be considered, since if a void 
is encountered, all of the drilling flu- 
ids and the hole may be lost. If the 
hole could be saved, expensive remedial 
actions such as casing through the mine 
opening might be required. 

All of the geologic situations de- 
scribed in figure 13 would also have 
serious effects on horizontal drilling 
activities (11). Sand channels are par- 
ticularly troublesome because of their 
large size and slow rate of penetration 
with the horizontal drilling equipment. 
Intercepting an abandoned mine with a 
horizontal borehole could be hazardous if 
the mine was full of water (or gas) which 
flowed uncontrolled into the mine work- 
ings from which the hole was being 
drilled. 

Since coalbed discontinuities can seri- 
ously affect the successful completion of 
resource confirmation core holes as well 
as vertical and horizontal methane drain- 
age boreholes, they should be evaluated 
as part of the feasibility studies for a 
specific project area or drill site. 
While it is not possible to precisely 
locate all discontinuities (especially 
the smaller ones) before drilling a 



particular site, basic geologic mapping 
techniques can project probable areas of 
occurrence if sufficient data are avail- 
able. It is also possible to estimate 
the probability of encountering coalbed 
discontinuities by statistically evalu- 
ating data from mines in adjoining areas 
(12). Impermeable coalbed discontinu- 
ities are also important from a mine ven- 
tilation standpoint since they can iso- 
late large volumes of gas that can be 
liberated suddenly in high volumes when 
penetrated by a mine entry. 

Multiple Coalbed Reservoirs 

Multiple coalbed reservoirs can be 
attractive for a resource recovery 
program using vertical boreholes; how- 
ever, their thickness and distribution 
must be amenable to efficient well com- 
pletion practices. Completions of multi- 
ple coalbed reservoirs may also have 
applications in mining when more than one 
coalbed is to be mined or where gas from 
surrounding coalbeds may migrate to the 
workings in the coalbed being mined. 

In general, it is preferable to have 
the coalbed completely exposed (open-hole 
completion) to the wellbore for the most 
efficient gas production (23). The com- 
pletion of multiple coalbeds, if they are 
distributed over a large interval in the 
well, can necessitate the installation of 
casing through the upper coalbeds, which 
is less desirable. In well A (fig. 14) 
two thick coalbeds have been encountered 
at the bottom of the hole. Assuming that 
both coalbeds have sufficient gas to war- 
rant completion, both could probably be 
completed open hole with the casing set 
above the upper coalbed. Depending on 
the actual distance between the two coal- 
beds and the competence and condition of 
the intervening rock unit, each coalbed 
could be stimulated separately or at the 
same time. Separate treatments would be 
desirable to increase the probability cf 
getting a good stimulation treatment in 
each coalbed. If both coalbeds were 
treated at the same time, there would be 
that the treatment would only 
the coalbeds, in spite 
designs to avert that 



a chance 
enter one of 
of treatment 



18 



Well A 



Well B 










tS i—r 


i i —r 




i ■ — 


1 , i ' 


i i i 


i i 




-V — rh- 


'^""i ■ ! 


1 1 




ii i — 




1 1 ■ i'i 






* ■ * 


II II 




1 1 1 


i i 



























































































































Coal 



nzi 



l 1, 1 



LEGEND 



Limestone, sandy shale 



Sandy shale 



FIGURE 14. - Examples of multiple coalbed reservoirs. 



situation. A good stimulation treatment 
is critical for efficient production of 
methane from a coalbed (23)» The addi- 
tional cost for separate treatments must 
be balanced against the potential for 
incomplete stimulation of coalbeds in a 
zone treatment. 

Well B (fig. 14) represents an unde- 
sirable situation involving multiple 
coalbeds. Instead of one or more thick 
coalbeds being encountered, all of the 
coalbeds are thin, and they are spread 



over a large interval in the well. Even 
though collectively all of the coalbeds 
in well B may contain a volume of gas 
equal to or greater than that of a single 
thick gassy coalbed, the completion cost 
will probably be high and the production 
potential low. If the interval contain- 
ing the coalbeds is large and the rock 
between the coals is subject to deteri- 
oration and sloughing, the upper coalbeds 
would have to be cased to prevent the 
hole from filling in. 



19 



When casing is installed through a 
coalbed that is to be completed for pro- 
duction, communication between the coal- 
bed reservoir and the wellbore must be 
established either using conventional 
perforations or preferably by slotting 
(6, 28). The perforations or slots must 
be precisely located at the coalbed in- 
terval to have a reasonable chance of a 
successful completion. The thinner the 
coalbed, as in well B (fig. 14), the 
greater the chance of missing the coalbed 
when an attempt is made to perforate or 
slot. 



The coalbeds in well B (fig. 14) would 
probably have to be completed with three 
separate stimulation treatments. The 
upper three coalbeds would be grouped as 
a zone, and the bottom two coalbeds would 
be stimulated individually. The produc- 
tion potential of the coalbeds in well B 
would probably not justify the high cost 
of this completion program, or even the 
cost of the well itself. 



REGIONAL CONSIDERATIONS 



Regional considerations for coalbed gas 
potential are essentially an expansion 
and correlation of information gained 
from site-specific evaluation procedures. 
The regional considerations, like the 
site-specific considerations, have both 
mine safety and resource production po- 
tential applications (7). 

Calculation of an Area's In-Place 
Gas Volume 

A calculation of the in-place gas vol- 
ume in an area and mapping of the gas 
distribution are primary regional con- 
siderations. To calculate the in-place 
gas volume, the following are needed: A 
means of estimating the gas content that 
is related to mappable parameters (direct 
method gas contents versus depth, 
fig. 10), and maps of the parameters 
(depth of the coalbed in the area, and 
coal thickness, fig. 15). 

Once the appropriate maps have been 
constructed and gas contents from coal 
samples versus depth have been graphed, 
the actual calculation of the in-place 
gas volume is quite simple. The data 
from the Mary Lee Coal Group in the 



Warrior Basin of Alabama will be used as 
an example. The overburden map (fig. 15) 
has been drawn with a 152-m (500-ft) con- 
tour (depth) interval. The coal isopach 
(thickness) map has been superimposed on 
the overburden map so that the volume of 
coal in each 152-m (500-ft) depth inter- 
val can be calculated. 

For estimation purposes, the gas con- 
tent of the median depth of each 152-m 
(500-ft) overburden interval (fig. 10) is 
used in the calculation as the average 
gas content of the interval. The gas 
content of the median depth of each in- 
terval is multiplied by the volume of 
coal in the interval to obtain the in- 
place gas volume. As an example, the 
gas content of the median depth of the 
305- to 457-m (1,000- to 1, 500-ft) Mary 
Lee overburden interval is 14.0 cm 3 /g 
(448 ft 3 /ton), and the coal volume is 
1,421 trillion kg (1,566 billion tons), 
which when multiplied yields 19.9 billion 
m 3 (702 billion ft 3 ) of gas for the in- 
terval. Similar calculations are made 
for each interval (table 6) , and the 
total in-place volume (52.3 billion m 3 
[1.8 trillion ft 3 ] for the Mary Lee 
Group) can be determined. 



20 



TABLE 6. - In-place gas volume for the Mary Lee Coal Group, Alabama 



Overburden 


Average gas content 


Gas in place 


m 


ft 


cm^/g 


ft-Vton 


Billion m* 


Billion ft* 


0-152 


0- 500 


0.5 


16 


0.8 


28 


152-305 


500-1,000 


9.2 


294 


15.0 


530 


305-457 


1,000-1,500 


14.0 


448 


19.9 


702 


457-610 


1,500-2,000 


15.5 


496 


11.9 


419 


610-762 


2,000-2,500 


15.7 


502 


4.7 


165 




52.3 


1.8 million 



N 






Jasper 



v^3 



WALKER COUNTY 



V-^ 



BLOUNT 
COUNTY 



y^ 






_K 



JEFFERSON COUNTY 











"-500- Overburden thickness, 
contour interval =500 ft 

— 8-" Coal thickness, 

contour interval = 4 ft 



40,000 ft 



12,000 m 



FIGURE 15. - Isopach of the Mary Lee Coal Group superimposed on the overburden 
isopach. 



The gas content information can be used 
in conjunction with the regional over- 
burden map to delineate areas of high in- 
place gas volumes (potentially bad for 
mining, good for commercial production) 
and low in-place gas volumes (potentially 
good for mining, bad for commercial pro- 
duction). Since the gas content of coal 
increases with depth, the deeper parts of 
the Warrior Basin, as delineated on the 
overburden map, have the highest poten- 
tial for large volumes of in-place gas. 
At a depth of 610 m (2,000 ft), every 
2.6 km 2 (square mile) of Mary Lee coal, 
1.8 m (6 ft) thick, would contain approx- 
imately 85 million m 3 (3 billion ft 3 ) of 
in-place gas. This volume of gas would 
probably be attractive for its resource 
production potential, but it would be a 
tremendous volume of gas to encounter in 
a mining operation. The gas content at a 
depth of 610 m (2,000 ft) from figure 10 
is approximately 15.7 cm 3 /g (502 ft 3 /- 
ton) , which when plotted on the graph of 
expected mine emissions (fig. 3) yields 
an estimate of over 96 m 3 (3,400 ft 3 ) of 
gas emissions from all sources (roof, 
floor, ribs, and pillars in addition to 
that actually contained in the volume of 
coal mined at the face) for each 907 kg 
(ton) of coal production. If possible, 
it would be preferable from the stand- 
point of the potential methane hazard 
(which of course is not the only con- 
sideration for locating a mining opera- 
tion) to locate in the areas of lower in- 
place gas volumes. 



21 



The general regional estimates of in- 
place gas volumes, as calculated by the 
procedure described above, can be a valu- 
able indicator of areas to be seriously 
considered for commercial gas production. 
Previous studies (4, 9-10, 14 , 16 , 25 , 
30-31) have estimated the in-place gas 
volumes for several of the gassiest coal- 
beds and coal-bearing formations in the 
United States (table 7), and several es- 
timates of the in-place coalbed gas vol- 
umes for the entire United States have 
been made (table 1). These estimates are 
only valuable if used with an understand- 
ing of their true meaning and signifi- 
cance. It is very important to realize 
that these values are for in-place gas 
volumes and do not represent the volume 
of gas that can physically and/or econom- 
ically be recovered from coalbed gas 
drainage systems. 

The percentage of in-place gas that can 
physically be removed from a coalbed is 
presently unknown and will probably be 
different for each coalbed and even for 
different areas of the same coalbed. It 
is probable that the volume of gas that 
is residual gas in the direct method test 
will probably not flow to a wellbore and 
perhaps will not flow into a mining oper- 
ation. Economically, it is unlikely that 
gas that is at low concentrations, as in 
the shallow portions of coal basins and 
in the low-rank coals, will ever be cap- 
tured for utilization. 



TABLE 7. - In-place gas volumes of selected U.S. coalbeds 



Coalbed or formation, and 


Area 


In-place gas volume 


State 


km^ 


mi^ 


Billion m 3 


Trillion ft 3 


Mesaverde Fm. (32), (Southern 
Piceance Basin), Colorado... 
Mesaverde Fm. (4), (Sandwash 

Vermejo Fm. (31), Colorado... 
Pittsburgh (9), Pennsylvania 


4,079 

1,072 

2,161 

464 

3,367 
715 

1,554 

1,295 
518 


1,575 

414 
835 
179 

1,300 
276 

600 

500 
200 


887.0 

396.7 
52.3 
44.2 

42.5 
39.6-283.2 

31.1-42.5 

5.7-11.3 
2.8 


31.3 

14.0 
1.8 
1.56 

1.5 


Fruit land Fm. (16), Colorado. 
Lower Hartshorne (14), 


1.4-10.0 
1.1-1.5 


Upper Freeport (30), 


0.2-0.4 


Beckley (25), West Virginia.. 


.1 



22 



Additional Regional Considerations 

The other regional considerations for 
coalbed gas drainage activities are pri- 
marily related to geologic factors for 
selection of areas within a region for 
site-specific evaluation of proposed com- 
mercial gas recovery projects. The 
thickness of coalbeds can vary on a re- 
gional basis as well as locally, as dis- 
cussed previously. The thicker the coal- 
bed, the larger the reservoir for gas 
storage. Also, various coalbeds can 
appear and disappear independently of 
each other throughout a region. This is 
important if multiple zone completions of 
vertical wells are anticipated. For com- 
mercial ventures it is necessary to pick 
an area for potential development that 
has the optimum balance of gas content, 



coal thickness, and number and distribu- 
tion (vertical thickness of producing 
zone and distance between individual 
coalbeds) of producible coalbeds if mul- 
tiple completions are planned. 

Regional trends of water-bearing sands 
(water sands) should also be considered 
to determine where such sands may be in 
close association with coalbeds from 
which gas is to be extracted by vertical 
wells. If water sands are present, high 
volumes of extraneaous water may be pro- 
duced, perhaps indefinitely, without 
dewatering the coalbed, and with little 
if any gas production. The type and cost 
of completion procedures for vertical 
wells can be seriously affected by the 
presence of water sands in a prospective 
producing zone. 



CONCLUSIONS 



Coalbeds can contain appreciable quan- 
tities of methane, the removal of which 
may be desirable for mine safety and/or 
energy resource utilization purposes. 
The direct method test can be used to 
determine the gas contents of coalbeds at 
specific sites of future mining opera- 
tions or potential resource recovery and 
utilization systems. Geologic evalu- 
ation, including a determination of the 
potential of encountering coalbed discon- 
tinuities, is an important consideration 
when locating methane drainage drilling 
sites. Maximum information can be ob- 
tained from a resource confirmation ex- 
ploratory hole if a continuous wire line 
core is obtained. If wire line coring is 
not possible, a twinned-hole approach is 
the second choice. 



Regional considerations influencing the 
gas potential of coalbeds are primarily 
related to the calculation of the total 
in-place gas for a coalbed (or group of 
coalbeds) in an area. Caution must be 
used not to confuse the in-place gas vol- 
umes with recoverable volumes. The dis- 
tribution of the gas volumes in a region 
can be used to delineate areas of high 
in-place gas volumes where mining may be 
adversely affected but resource recovery 
and utilization may be enhanced. Region- 
al mapping of geologic trends such 
as coal thickness, number of coalbeds, 
and water sands can aid in delineating 
areas with the highest potential for 
commercial production of methane from 
coalbeds. 



REFERENCES 



1. American Geological Institute. 
Glossary of Geology. Falls Church, Va. , 
1974, p. 201. 

2. American Society for Testing and 
Materials. Standard Specification for 
Classification of Coals by Rank. D388 in 
1977 Annual book of ASTM Standards: 
Part 26, Gaseous Fuels; Coal and Coke; 
Atmosphere Analysis. Philadelphia, Pa., 
1977, pp. 214-218. 



3. Ancil, K. L. , S. Lambert and F. S. 
Johnson. Analysis of the Coalbed Degasi- 
fication Process at a Seventeen Well Pat- 
tern in the Warrior Basin of Alabama. 
SPE/DOE 8971 in Proc. 1st Ann. Symp. 
on Unconventional Gas Recovery. Soc. 
Petrol. Eng. AIME and U.S. Dept of 
Energy, Pittsburgh, Pa., May 18-20, 1980, 
pp. 355-370. 



23 



4. Boreck, D. L. , C. M. Tremain, L. 
Sitowitz, and T. D. Lorenson. The Coal- 
bed Methane Potential of The Sandwash 
Basin, Green River Coal Region, Colorado. 
Colo. Geol. Survey Open-File Rept. 81-6, 
1981, 25 pp. 

5. Cervik, J. Behavior of Coal-Gas 
Reservoirs. BuMines TPR 10, 1969, 10 pp. 

6. Deul, M. , and A. G. Kim. Methane 
Drainage — An Update. Min. Cong. J. , 
v. 64, No. 7, July 1978, pp. 38-42. 

7. Diamond, W. P. Evaluation of the 
Methane Gas Content of Coalbeds: Part of 
a Complete Coal Exploration Program for 
Health and Safety and Resource Evalua- 
tion. Proc. 2d Internat. Coal Explora- 
tion Symp., Denver, Colo., Oct. 1-4, 
1978, v. 2, pp. 211-222. 

8. Diamond, W. P., and J. R. Levine. 
Direct Method Determination of the Gas 
Content of Coal: Procedures and Results. 
BuMines RI 8515, 1981, 36 pp. 

9. Diamond, W. P., and G. W. Murrie. 
Methane Gas Content of the Pittsburgh 
Coalbed and Evaluation of Drilling Re- 
sults at a Major Degasif ication Installa- 
tion. Pres. at Northeastern Sec. Ann. 
Meeting, Geol. Soc. America, Binghamton, 
N.Y., Mar. 31-Apr. 2, 1977, abs. with 
Programs, v. 9, No. 3, February 1977, 
p. 256. 

10. Diamond, W. P., G. W. Murrie, and 
CM. McCulloch. Methane Gas Content of 
the Mary Lee Group of Coalbeds, Jeffer- 
son, Tuscaloosa, and Walker Counties, 
Ala. BuMines RI 8117, 1976, 9 pp. 

11. Finfinger, G. L. , L. J. Prosser, 
and J. Cervick. Influence of Coalbed 
Characteristics and Geology on Methane 
Drainage. SPE/DOE 8694 in Proc. 1st Ann. 
Symp. on Unconventional Gas Recovery, 
Soc. Petrol Eng. AIME and U.S. Dept. of 
Energy, Pittsburgh, Pa., May 18-20, 1980, 
pp. 319-324. 

12. Houseknecht, D. H. Probability 
of Encountering Coalbed Discontinui- 
ties During Vertical and Horizontal 



Borenole Drilling. 
1982, 21 pp. 



BuMines RI 8665, 



13. Iannacchione, A. T. , and D. G. 
Puglio. Geology of the Lower Kittanning 
Coalbed and Related Mining and Methane 
Emission Problems in Cambria County, Pa. 
BuMines RI 8354, 1979, 3 pp. 

14. . Methane Content and Geol- 
ogy of the Hartshorne Coalbed in Haskell 
and Le Flore Counties, Okla. BuMines RI 
8407, 1979, 14 pp. 

15. Irani, M. C, J. H. Jansky, P. W. 
Jeran, and G. L. Hassett. Methane Emis- 
sion From U.S. Coal Mines in 1975, a Sur- 
vey. A Supplement to Information Circu- 
lars 8558 and 8659. BuMines IC 8733, 
1977, 55 pp. 

16. Kelso, B. S., S. M. Goolsby and C. 
M. Tremain. Deep Coalbed Methane Poten- 
tial of the San Juan River Coal Region, 
Southwestern Colorado. Colo. Geolo. Sur- 
vey Open-File Rept. 80-2, 1980, 56 pp. 

17. Kim, A. G. The Composition of 
Coalbed Gas. BuMines RI 7762, 1973, 
9 PP. 

18. . Estimating Methane Con- 
tent of Bituminous Coalbeds From Adsorp- 
tion Data. BuMines RI 8245, 1977, 
22 pp. 



19. . 

the Origin and 

Gas. BuMines RI 8317, 1978, 18 pp. 



Experimental Studies on 
Accumulation of Coalbed 



20. Kissell, F. N. , and J. C. Edwards. 
Two-Phase Flow in Coalbeds. BuMines RI 
8066, 1975, 16 pp. 

21. Lambert, S. W. , and M. A. Trevits. 
Effective Placement of Coalbed Gas 
Drainage Wells. DOE RI-PMOC-2(78) , 1978, 
16 pp. 

22. . Methane Drainage: Experi- 
ence With Hydraulic Stimulation Through 
Slotted Casing. BuMines RI 8295, 1978, 
16 pp. 



24 



23. Lambert, S. W. , M. A. Trevits, and 
P. G. Steidl. Vertical Borehole Design 
and Completion Practices To Remove Meth- 
ane Gas From Mineable Coalbeds. DOE/C 
MTC/TR-80/2, 1980, 163 pp. 

24. National Petroleum Council. Un- 
conventional Gas Sources, Volume II, Coal 
Seams. June 1980, p. 14. 

25. Popp, J. T., and C. M. McCulloch. 
Geological Factors Affecting Methane in 
the Beckley Coalbed. BuMines RI 8137, 
1976, 35 pp. 

26. Price, H. S. New Advances in 
Coalbed Methane. Proc. 1st Internat. Gas 
Res. Conf., Gas Res. Inst., Am. Gas 
Assoc, and U.S. Department of Energy, 
Chicago, II., June 9-12, 1980, 112 pp. 

27. Puglio, D. G. , and A. T. Iannac- 
chione. Geology, Mining, and Methane 
Content of the Freeport and Kittan- 
ning Coalbeds in Indiana and Surrounding 
Counties, Pa. BuMines RI 8406, 1979, 
35 pp. 

28. Skow, M. L. , A. G. Kim, and M. 
Deul. Creating a Safer Environment in 
U.S. Coal Wines: The Bureau of Mines 
Methane Control Program, 1964-79. Bu- 
Mines Impact Report, 1980, 50 pp. 



29. Stach, E., M.-Th. Mackowsky, M. 
Teichmuller, G. H. Taylor, D. Chandra, 
and R. Teichmuller. Stach* s Textbook of 
Coal Petrology, trans, and rev. by D. G. 
Murchison, G. H. Taylor, and F. Zierke. 
Gebruder Borntraeger, Berlin and Stutt- 
gart, Germany, 1975, 428 pp. 

30. Steidl, P. E. Geology and Methane 
Content of the Upper Freeport Coalbed in 
Fayette County, Pa. BuMines RI 8226r, 
1977, 17 pp. 

31. Tremain, C. M. The Coalbed Meth- 
ane Potential of the Raton Mesa Coal 
Region, Raton Mesa, Colorado. Colo. 
Geol. Survey Open-File Rept. 80-4, 1980, 
48 pp. 

32. Tremain, C. M. , and S. K. Aumil- 
ler. Deep Coalbed Methane Potential of 
the Southern Piceance Basin, Colorado. 
Colo. Geol. Survey Open-File Rept. 82-1, 
1982, 42 pp. 

33. U.S. Department of Energy. Na- 
tional Energy Plan II, A Report to the 
Congress Required by Title VIII of the 
Department of Energy Organization Act. 
May 1979, p. 95. 



INT.-BU.O F MIN ES, PGH..PA. 26436 




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